This project attempts to understand the biochemical basis of T cell activation and the role of Ir genes in the regulation of this immune response. During the past year we have been successful at raising monoclonal antibodies specific for the antigen-specific receptor on MHC-restricted T cell clones. These antibodies inhibit antigen-induced activation of the clones and immunoprecipitate a disulfide-bonded heterodimer composed of one chain of approximately 40,000-42,000 daltons and the other of approximately 45,000-48,000 daltons. Future studies will be directed toward a further biochemical characterization of these molecules. In a separate set of experiments we discovered that normal B cell populations can function as antigen-presenting cells provided they are unirradiated. In addition, activation of T cells for cell division and B cell recruitment were found to involve separable cellular pathways.